In the present invention, the light source is provided with an external cavity ended by a reflecting device towards the source of the light emitted by this source.
This source may be a laser, such as a gas laser whose cavity has been elongated.
The source may also be a semiconductor laser whose face, which is on the side of the reflecting device, is treated by a coating.
The device of the invention is then designed in such a way as to be inserted into the cavity of the semiconductor laser and makes it possible to continuously tune the emission frequency of this semiconductor laser on a plurality of spectral ranges which may cover the width of the spontaneous emission spectrum of the semiconductor laser.
One of the objectives sought in linkages by optical fibers is the tuneability of the emission frequency of sources (laser diodes) used in such linkages and in particular so as to overcome the problems of adjustment and the drift of the wavelengths of the sources. Thus, in wavelength multiplexing, the theoretically unlimited number of channels is, in practice, limited by the wavelength drifts of the laser diode and by the dispersion of the emission parameters of the laser diodes at the time these laser diodes are produced. Because of this, most wavelength multiplexed telecommunications systems are currently limited to about twelve channels between 800 nm and 1300 nm owing to the lack of reliable devices able to finely adjust the emission wavelength of the laser diodes.
The tuneability of the emission frequency of laser diodes also constitutes a key problem in long-distance optical coherent links. In this type of link, the frequency of the emitting source needs to be tuned to that of the local oscillator so as to carry out a demodulation by optical heterodyning. Similarly, in local multicolored networks, the optical multiplexing of several channels on a given network is effected by allocating to each channel a particular optical frequency with the aid of laser diodes locked onto predetermined frequencies.
At each end of the network, demultiplexing is carried out by heterodyning with the aid of a laser diode (behaving as a local oscillator) whose frequency the tuned to one of the bearer frequencies. The installing of monofrequency semiconductor lasers with a continuously tuneable frequency is therefore still essential for local networks with a high multiplexing density.
Other applications also require fine and continuous tuneability of the emission frequency of the laser diodes:
In optical metrology, certain measuring techniques implementing, for example, the "speckle" phenomena at several wavelengths use the phenomena of multiwavelength interference so as to determine the topography of a surface or the state of this surface.
In molecular spectroscopy, the analysis of the emission or absorption spectrum of molecules illuminated by a luminous wave with a rapidly variable frequency may yield information concerning their dynamic or transitory behaviour.
The use of a laser diode having a continuously tuneable emission wavelength has also recently been proposed for studying wavelength chaos, multistability and bistability phenomena (IEEE Journal of Quantum Electronics, 23, 2, 1987, pp. 153-157).
Generally speaking, the continuous tuneability of the emission frequency of a laser diode is obtained by modifying the current traversing the junction point of this laser diode or by modifying the temperature of the laser diode. However, the frequency drift thus obtained remains relatively small (several hundreds of MHz/mA and about 1GHz/.degree.C.) and proves insufficient for the above-mentioned applications.
Another method consists of using a laser diode mounted in the external cavity inside which an optical grating is inserted. In this respect, reference may be made to the documents (1) to (4) which, like the other documents cited subsequently, are mentioned at the end of this description.
It is also possible to consult the document (5).
The documents (1) to (4) describe various devices for the continuous tuning of the frequency of the external cavity laser diodes. By means of rotation, a diffraction grid makes it possible to obtain a monofrequency emission (with a line width of several tens of kHz) tuneable over an extended (30 nm) spectral range centered on 1300 nm or 1500 nm.
Continuous tuneability is obtained by modifying the optical length of the external cavity simultaneously with rotation of the grating contained in this cavity. This modification of the optical length of the cavity is obtained by using either the travel of the grating (documents (1) and (2)) or by piezoelectric translation devices (document (4)) or even by modifying the current traversing the junction point pn of the laser diode (document (3)). However, for certain applications, these techniques are limited by virtue of their low tuning speed, their sensitivity to the mechanical adjustments or by them being unable to pass from one frequency to another without passing through intermediate frequencies.
A tuneability obtained electrically rather than mechanically is more advantageous as it makes it possible to overcome the preceding drawbacks. One solution consists of monolithically integrating the tuneability function on a DFB or DBR laser from electrically controlled laser structures with separate sections. By controlling the current in each of the sections, it is possible to obtain a tuneable monomode emission. Such systems require fine control of the injection currents and also require extremely elaborated technology in order to embody the films forming such structures, such as those described, for example, in the document (6). Continuous tuning ranges of 380 GHz have been recently obtained in a 1500 nm window with line widths of between 20 and 100 MHz.
Another known technique making it possible to obtain an electrically tuneable emission frequency consists of using an electro-optical modulator mounted in the cavity of an external cavity laser diode. This type of device is derived from wavelength tuneable devices with dye lasers described, for example, in the documents (7) to (11) and in which tuneability is effected by mode jumps.
Thus, the documents (10) and (11) describe an electro-optical tuning system comprising a KD P crystal with a 0.degree. Z section inserted into the cavity of an external cavity laser diode and making it possible to obtain a tuning by mode jumps on a range of 4 nm in the 800 nm window by applying a voltage of 6 kV to the crystal.
Another known system making it possible to obtain a tuning by mode jumps is described in the document (12).
Another approach put forward in the document (13) consists of using an external cavity laser diode coupled to an electro-optical spectral filter integrated onto an LiNb0.sub.3 substrate with a section X with propagation of the light along the axis Y. The electro-optical filter comprises a double set of electrodes. A first set of interdigitalized electrodes allows for tuning of the emission frequency by mode jumps on an extended range by introducing a selective coupling between the propagation modes TE and TM extending into an optical guide embodied by diffusing Titanium in the substrate. The second set of electrodes induces a phase modulation on the radiation extending into the guide so as to carry out continuous tuneability over a narrow range.
In this known device, a frequency tuning on a range of 7 nm centered on 1500 nm is embodied by mode jumps by applying a voltage of 100 volts to the first set of electrodes. Continuous tuneability is effected on a plurality of ranges of 1 GHz with a line width of 60 kHz for a wavelength cavity equal to 8 cm.
A further solution making it possible to electrically tune the frequency of an external cavity laser diode consists of using two acousto-optical cells placed in the cavity, as described in the document (14). The tuning range extends over 35 nm, is centered on 1500 nm, but is effected by mode jumps.
It is also possible to consult the document (15) as regards tuning with the aid of acousto-optical cells.